This adds:
- A global Date object (with `length` property and `now` function)
- The Date constructor (no arguments yet)
- The Date prototype (with `get*` functions)
Instead of implementing every native function as a lambda function,
use static member functions instead.
This makes it easier to navigate the code + backtraces look nicer. :^)
Native functions now only get the Interpreter& as an argument. They can
then extract |this| along with any indexed arguments it wants from it.
This forces functions that want |this| to actually deal with calling
interpreter.this_value().to_object(), and dealing with the possibility
of a non-object |this|.
This is still not great but let's keep massaging it forward.
